Hi Steor,
Like you i have just acquired a tri foiler,and am currently rebuilding it.
The hole you ask of is (on my boat)250mm up from the bottom of the rudder, or 50 mm up from the bottom "fence".
Hope that is of some use to you.
I have been contacting Dan Ketterman for information and parts...here is his email.
DKetterman@hobiecat.com

Thank you very much for your reply: in the meanwhile I found the log inlet.
Today for the first time I ride my Trifoiler onto foils: Very exciting experience!!
I think I need time to do a good set up.
For sure the rudder was not correctly tuned and so the foils flew to high.
Then I broke the padeye of the stb shock absorber.
The wind was going up to fast and the waves too so, I decided to come back.
However I happy to find you in the forum: keep in touch and tell about your experience. It can be helpfull for both of us.
Ciao
Stefano